Figure 1-16.Header joist.placed in sections. Solid girders must be measured andcut so that the ends fall over the center of a post. Built-upgirders should be placed so their outside joints fall overthe posts (fig. 1-13).FLOOR JOISTSIn platform framing, one end of the floor joistrests directly on the sill plate of the exteriorfoundation wall or on the top plate of a framed outsidewall. The bearing should be at least 1 1/2 inches. Theopposite end of the joist laps over or butts into aninterior girder or wall. The size of joist material (2 by6, 2 by 10, 2 by 12, and so forth) must be chosen withconsideration for the span and the amount of load tobe carried. The foundation plan usually specifies thejoist size, the spacing between joists, and whatdirection the joists should travel.The usual spacing of floor joists is 16 inches OC.Floor joists are supported and held in position overexterior walls by header joists or by solid blockingbetween the joists. The header-joist system is used mostoften.HeaderHeader joists run along the outside walls. Three 16dnails are driven through the header joists into the endsof the common joists, as shown in figure 1-16. Theheader and joists are toenailed to the sill with 16d nails.The header joists prevent the common joists fromFigure 1-17.Lapped joists.rolling or tipping. They also help support the wall aboveand fill in the spaces between the common joists.LappedJoists are often lapped over a girder running downthe center of a building. The lapped ends of the joistsmay also be supported by an interior foundation orframed wall. It is standard procedure to lap joists the fullwidth of the girder or wall. The minimum lap should be4 inches. Figure 1-17 shows lapped joists resting on asteel girder. A 2- by 4-inch plate has been bolted to thetop of a steel beam. The joists are toenailed into the plate.Solid blocking may be installed between the lapped endsafter all the joists have been nailed down. Anothersystem is to put in the blocks at the time the joists areplaced.DoubleJoists should be doubled under partitions running inthe same direction as the joists. Some walls have waterpipes, vent stacks, or heating ducts coming up from thebasement or the floor below. Place bridging betweendouble joists to allow space for these purposes(fig. 1-18).CantileveredCantilevered joists are used when a floor or balconyof a building projects past the wall below, as shown infigure 1-19. A header piece is nailed to the ends of the1-8